Toyota 2001 Rav4 Automatic 69000 mile gear changing problem

PSIRISH

2001 TOYOTA RAV4

I recently had taken my car to an authorised repair shop since my car's "Check Engine" lights came on. The shop told me that my car's oxygen sensors need to be replaced. Within days of that change, my car started giving some problems. When in slow speed, I try to accelarate, sometimes, even though I have the accelarator pressed in, and I can see the RPM meter increasing, the car does not speed up, after a moment, it does speed up - but with a jerk. This happens at speeds around 20-25. It is otherwise functioning normally at high speeds. I have around 69000 miles on my car. I have been very regular in getting my car serviced every 5000 miles from an authorised repair shop. What could be the problem?